Chemistry of Lipid Oxidation

V. Comparison of Triplet and Singlet Oxygen Oxidation..................................................... 309 VI. Flavor Properties of Volatile Compounds from Oil Oxidation ........................................ 310 VII. Singlet Oxygen Oxidation of Soybean Oil ....................................................................... 310 VIII. Singlet Oxygen Quenching Mechanisms.......................................................................... 312

A. Quenching Mechanism of Carotenoids..................................................................... 313 B. Quenching Mechanisms of Tocopherols................................................................... 316

IX. Determining Quenching Mechanisms............................................................................... 316 References ..................................................................................................................................... 318

Lipid oxidation causes nutritional losses and produces undesirable flavor, color, and toxic compounds, which make foods less acceptable or unacceptable to consumers. Lipid oxidation can occur by either diradical triplet oxygen or nonradical singlet oxygen. Triplet oxygen oxidation has been extensively studied during the last 70 years to improve the oxidative stability of foods. However, triplet oxygen oxidation could not fully explain the initiation step of lipid oxidation [1]. Rawls and Van Santen [2] suggested that singlet oxygen is involved in the initiation of lipid oxidation because nonradical and electrophilic singlet oxygen can directly react with double bonds of food components without the formation of free radicals. Singlet oxygen oxidation of lipids is very significant because the rate of singlet oxygen oxidation is much greater than that of triplet oxygen oxidation. Singlet oxygen rapidly increases the oxidation rate of foods even at very low temperatures [2]. Singlet oxygen oxidation also produces new compounds, which are not found in ordinary triplet oxygen oxidation in foods [1,3].